The electronic circuit board assembly mounting art is replete with varied mechanical layouts and techniques for permitting the insertion or plug-in and removal of pluralities of stacks of electronic boards or cards within confined frames, cages and housings.
With the use of large pluralities or banks of boards such as memory, CPU or I/O boards in computer systems, or line interface boards, switch fabrics, and control processors in networking equipment, problems attendant upon the cooling of proximally mounted boards requiring large amounts of power have arisen, along with the requirement for benign signaling environments for very high speed, low voltage signals. These very high-speed circuits, moreover, can generate strong electromagnetic radiation fields that must also be contained with the enclosure system. As more boards are required, they must be mounted closer and closer together, greatly increasing the airflow impedance and decreasing the allowable maximum component height on each board. This creates difficult cooling problems such as hot spots and dead zones on the boards, and this is additionally complicated by the increased power requirement for today's high-speed integrated circuits. These high-power, high-speed circuits require large amounts of airflow volume and very small enclosure openings to contain electromagnetic radiation; and these two requirements, unfortunately, are diametrically opposed.
Most current computers and networking equipment are constrained to be packaged in fixed width cabinets, such as 19-inch EIA racks. Designers have thus been forced to find ways of packaging more and more boards into such a fixed width chassis. One obvious method is to decrease the space between boards hence increasing the number of vertical boards in a chassis. Beyond sixteen boards, however, this become extremely difficult as the pitch of the boards falls below 1-inch, leaving little height on boards for taller components (and heat sinks) and dramatically increasing the airflow impedance, thereby requiring very powerful blower systems.
Another approach currently being used is to mount some boards vertically and others horizontally, either above or below the vertical boards. While many networking vendors are employing this approach, it has serious drawbacks. Such an approach, indeed, requires cooling air to be forced in two directions, vertically and horizontally within the chassis or housing, with the horizontal air intake and exhaust developing convection effects, and deleteriously exhausting air from the sides into adjacent equipments as in central offices and the like; and with side-exhausted air becoming pulled in by the blowers or fans providing the vertical cooling air for the vertical boards. When air is forced to turn corners or bend, however, energy is wasted and thermal performance is sacrificed. Further drawbacks of this approach revolve around the distance between the horizontal boards and the vertical boards. To provide an adequate air intake plenum, this space should be as large as possible. The larger the space between the boards, however, the greater the distance of the electrical path. Since many vendors are installing their switch fabric boards horizontally, this increases the distance between the I/O boards and the switch fabric.
Through the novel mounting approach of the present invention, on the other hand, all boards may be vertically mounted and with high densities in standard width enclosures; all airflow is strictly vertical, a low impedance airflow path is maintained; and a power distribution scheme is used to separate high current noisy supply voltages from low voltage sensitive interface logic, and with tight electromagnetic radiation containment.